Kompfner



R. KOMPFNER APPARATUS UTILIZING SLALOM FOCUSING Aug. 11, 1959 4 Sheeis-Sheet 1 Filed June 10, 1955 lNl/EN TOR R. KOMPFNER 7 ATTORNEY 1959 Y R. KOMPFNER 2,899,597

APPARATUS UTILIZING SLALOM FQCUSING Filed June 10, 1955 4 Sheets-Sheet 4' 'lNl ENTok KO MPFNER A 7' TORNE Y APPARATUS UTILIZING SLALOM FOCUSING Rudolf Kompfner, Far Hills, NJ., assignor to Bell Tele- .phone Laboratories, Incorporated, New York, N.Y., a corporation of NewYork Application June 10, 1955, Serial No. 514,424

11 Claims. (Cl. 3 153.6)

This invention relates to electronic apparatus and more particularly to devices which employ an electron beam whose path of travel is controlled in accordance with input intelligence.

In a copending application Serial No. 514,423, filed June 10, 1955, now US. Patent No. 2,857,548, issued October 21, 1958, by R. Kompfner and W. H. Yocom, there are set forth novel principles applicable to the electrostatic focusing of an electron beam. In particular, it is there set forth that there is associated with a longitudinally-spaced array of transversely-extending parallel elements, which are maintained at a positive D'.-C.- potential'with respect to an electron source, a pair of singular equipotential surfaces which are characterized in that each of-such surfaces provides a stable trajectory for elec trons emitted from the electron source and injected substantially on either of these surfaces with a correct velocity. These singular equipotential surfaces wind, with progress in the longitudinal direction, sinuously between adjacent elements of the array in the manner reminiscent of askier slalomingpast a linear array of flag markers, and for this reason, it is thought descriptive to characterize focusing of this kind as slalom focusing. V

The present invention relates particularly to theextens'ionof such focusing principles to devices in which the trajectory of an electron beam is modifiedin a useful manner and in a prescribed fashion under the influence of control intelligence. More particularly, it is based to a considerable extent on the discovery of the additional principle that the electron trajectory in a focusing system of the kind described may be modified in a stable fashion by a' change in the potential along one or more of 'the elements forming the linear array used to establish the singular equipotential surface defining the beam trajectory. Essentially, the modification of the potential on such element or elements of the array results in a change of the path of the singular equipotential surface serving asthe path of electron flow and the beam flows stably along-the new path.

In particular, it is possible to make the two singular equipotential surfaces associated with a linear array close on one another to form a continuous loop so that electrons originally moving away from the electron source along one such surface will, after passing a prescribed element of the linear array, be turned back for flow along the other such surface towards the electron source.

Alternatively, additional arrays of elements maybe provided which branch out from the main array and electrons moving along a singular equipotential surface set up by the main array may, under the influence of control intelligence, be diverted to flow along a singular equipotential surface set up by a selected one of the branch arrays.

Several illustrative embodiments will be described, each of which is characterized by provision for diverting, under the'control of potential changes on elements of the electrode system, the electron beam into a selected one of a plurality of possible paths, each possible path correspond 2 ingto a singular equipotential surface set up by an electrode array.

In a storage tube embodiment of the invention, a linear array of elements establishes a pair of singular equipotential surfacesalong-oneof which'is injected an electron beam 'WlthTthB correct velocity for-flow therealong. By modifying thepotentials on a spaced pair of the elements'of'the array'iin a fashion to be described, theelectron beam is made to circulate for a controllable time interval alonga closed path defined by the "pair of singular equipotential surfaces closing upon themselves to form acontinuous 1oop,"and in this manner there is controlled the time of transit in traveling from the electron source to the collector.

lnta switching tube embodiment of the invention, there is provided a mainarray of elements along whose length branch-outa plurality of auxiliary arrays with each of which there is associated a separate target. Interconnection between the electron source and a selected target is eifected by control of the potentials on the elements of the main and auxiliary arrays to provide a unique continuous singular equipotential surface between the electron source and the selectedtarget.

The-invention will be better understood from the following more detailed description taken in conjunction withthe accompanying drawings, in which:

Fig. 1 shows in perspective in simplified form an'electron beam system including an array of elements and the two singular equipotential surfaces associated therewith which serve as stable electron trajectories in accordance with the principles of slalom focusing described in the previously identified copending application of R. Kompfner and W. H. Yocom;

Fig. 2 shows as an illustrative embodiment of the in vention in a perspective cut-away view a storage tube utilizing a linear array of conductive elements for establishing singular equipotential paths which serve as electron trajectories;

Figs. 3A through 3D are sketches illustrating how the electron trajectories maybe modified in the storage tube shown in Fig. 2 under the influence of control potentials applied to elements in the array;

Fig. 4 shows schematically in a transverse sectional view as another illustrative embodiment of the invention a switching tube utilizing a two dimensional array of conductive elements; 7

Fig. 5 shows schematically as another illustrative embodiment of the invention a fragment of binary switching arrangement; and V Fig. 6 shows schematically, as still another illustrative embodiment of the invention, a fragment of a switching tube which is based on decimal-type switching.

With reference now more particularly to the drawings, in the electron beam system 10 shown in perspective in Fig. l, a plurality of conductive elements 11, typically wires,,are arranged to form a linear longitudinal array. Each of the elements extends transversely and is supported at its two ends by supports not shown. Spaced on opposite sides of the array and extending parallel thereto are the conductive members 12 and 13, typically plates, forming conductive boundaries. As described in the aforementioned Kompfner-Yocom patent, if the conductive elements 11 are each maintained at like positive po tentials with respect to an electron source and to the boundary members-12 and 13, there will be set up a pair of singular equipotential surfaces, which are mirror images of one another with the plane of the array of ele ments as a reflection plane. Each of these singular equipotential surfaces winds sinuously between successive elements in the manner shown. It is characteristic that electrons injected substantially onto either one of these surfaces with a correct velocity will have a stable trajectory along such surface. Electrons are shown flowing along one of these surfaces by stippling. There is also shown the complementary other singular equipotential surface. In particular, correct velocity may be readily achieved by accelerating the electrons emitted from the electron source by a potential equal to the potential of the singular equipotential surface. To this end, in Fig. 1 there is shown schematically a filamentary electron emissive cathode 14 positioned at the line interaction of the two singular equipotential surfaces and surrounding the cathode is an accelerating anode 15 which is provided with a transversely extending slit to permit egress of electrons emitted by the cathode on the singular equipotential surface which is to serve as the path of electron flow. A collector is positioned at the opposite end of the array between a pair of conductive elements. The collector comprises an inner collecting element 16 which is surrounded by a collector housing member 17 which is slit along one face corresponding to the region of interception of the singular equipotential surface along which the electrons are to be projected. By suitable connections (not shown herein) the cathode 14 and each of the boundary members 12 and 13 are maintained at ground, or reference potential level, the conductive elements 11 and the collector 16 are maintained at potentials considerably positive thereto, and the accelerating anode 15 and the collector housing 17 are maintained approximately at the potential of the singular equipotential surface, which is at a potential intermediate that of the reference level and the conductive elements. In copending application Serial No. 514,421, filed June 10, 1955, by J. S. Cook, R. Kornfner and W. H. Yocom there is described an improved arrangement for the injection of an electron beam on one of the two singular equipotential surfaces.

In operation, electrons emitted by the cathode are formed into a sheet beam which flows along a singular equipotential surface past the array of elements and is finally collected by the collector.

In Fig. 2 there is shown schematically and in perspective a variable delay or storage tube 20 which incorporates an electron beam system of the kind shown in Fig. l. The various tube elements are enclosed within an evacuated envelope 21, typically glass. The insulating supports 22 and 23 serve to support the linear array of elements 11 spaced from the conductive boundary members 12 and 13, and also are used to support in position the electron gun comprising the cathode 14 and the accelerating anode 15 and the collector assembly comprising the collector element 16 and the collector housing member 17. For illustrating schematically the manner in which operating potentials are to be applied to the tube elements, lead-in connections are shown to the bounding members 12 and 13 and ends of each of the various other electrode components of the electron beam system are shown extending through the insulating support 23 and out of the tube envelope. As shown schematically, the cathode 14 and the bounding members 12 and 13 are maintained at the reference or ground level corresponding to the negative terminal 31 of the voltage source 30. The accelerating anode 15 of the electron gun and the collector housing 17 are each maintained at an intermediate positive potential with respect to the reference level corresponding to the intermediate terminal 32 of the voltage source 39. The collector element 16 and all but two of the elements forming the linear array which are to serve as the control elements are maintained at a more positive potential with respect to the reference level corresponding to the most positive terminal 33 of the voltage source 30. The two control elements designated 11A and 11B of the array, which serve to modify the beam trajectory are connected, respectively, to separate input terminals A and B of the control unit 36, which also includes terminals C and D. Terminals C and D are connected, respectively, to the terminals 31 and 33 of the voltage source 30. In operation, the control unit 36 which is shown schematically as a pair of mechanical switches but ordinarily will comprise a pair of electronic switches serves to control the potentials applied to elements 11A and 11B. The role of the control unit will become more apparent from a description of the operation taken in conjunction with Figs. 3A through 3D.

In Fig. 3A, there is shown the storage tube when it is operating with the same positive potential on each of the elements forming the array. It is convenient to use plus and minus signs to denote the potential state of individual elements of the array. This corresponds to a state of the control unit in which each of its input terminals A and B is effectively connected to output terminal D. In this case, the electron beam shown by the solid line fiows uninterrupted past the array along the same singular equipotential surface until collected by the collector element (not shown), positioned at the end of the array.

In Fig. 33, there is shown the storage tube after the potential of element 11B has been lowered to a predetermined potential which corresponds to the interconnection of input terminal B to output terminal C in the control unit. In this case, the longitudinal flow of electrons beyond element 113 is inhibited and instead the electron beam flows back towards the source along the path shown corresponding to the other of the two singular equipotential surfaces associated with the linear array. The distance which these electrons will return towards the source from which they originated can be controlled by modifying in turn the potential on element 11A. In particular, if the potential on element 11A is reduced to the abovementioned predetermined potential as by the interconnection of terminal A to terminal C in the control unit, electrons proceeding from element 113 towards the element 11A will again be redirected towards the collector along the first singular equipotential surface, as shown in Fig. 30. Accordingly, with both elements 11A and 11B at the reduced potential, the trajectory of electrons trapped between elements 11A and 11B will be the closed path shown in Fig. 30 made up in one direction by one of the two singular equipotential surfaces and in the other direction by the other of the two singular equipotential surfaces.

By returning in turn the elements 11B and 11A to the same positive potential characteristic of the other elements of the array, corresponding to the interconnection in turn of terminals A and B to terminal D of the control unit the electron beam may be made to have again its original trajectory along only a single one of the singular equipotential paths between the electron source and collector. There can, accordingly, be introduced in the time of transit of an electron bunch between the source and the collector a delay fixed by the control of intelligence applied to the control unit. Signal information may be stored on the beam at the source in the form of electron bunches, as by pulsing appropriately an intensity control element of the gun. The time of transit of separate bunches may be delayed in a predetermined manner as described by information applied to the control unit.

The embodiment just described is intended merely to illustrate the general principles applicable. Various modifications will appear to a Worker in the art which extend the usefulness of a device of this kind. In particular, the number of elements of the array may be extended and control potentials may be applied to more than two of the elements for forming a number of trapping regions along the array of elements.

Additionally, special provision can be made for collecting electrons whose trajectory takes them back to the electron source. In the tube shown such electrons tend to be collected by the accelerating anode surrounding the cathode. Additionally, various expedients can be used to insure that individual electron bunches remain coherent during their delay, as by proper synchronization of th Y FiQ control potentials applied,

In Fig. 4, there is shownschematically a longitudinal crosssection ofaswitchingtube 40 which inmanyrespects resembles the tube shown in Fig. 2. The principal diiferenceis that the tube 40 provides a-number of parallel electronbeam systems between which interconnection can be made. i

To this end, there is provided a two dimensional array of conductive elements, each element of which can be supported between a pair of dielectric supports in the manner shown in Fig. 2. Of this two dimensional array, alternate linear arrays 41A through 41E are each used to set up an associated pair of singular equipotential surfaces of the kind previously described. Aligned with such arrays and at opposite ends thereof are the cathodes 42A through 42E and collectors 43A through 43E of the kind discussed above. Intermediate between the linear arrays 41A through 41B are the linear arrays of conductive elements 44A through 44D. Each element of this array advantageously is a wire extending between the two dielectric supports used for supporting the wires forming linear arrays of elements 41A through 41E. Advantageously, the conductive elements forming the linear arrays44A through 44D are staggered longitudinally in themanner shown with respect to the elements forming linear arrays 41A through 41E. Conductive plates 45and46 serve as conductive boundaries.

By way of illustration there are shown by solid lines four typical paths which an electron beam emitted by a cathode may take before being collected by a collector. The path which any beamwill take is controlled by the potentials appliedto the elements of'the linear array of elements 41A through 41E and 44A through 44D. The potentials maybe applied in the manner shown forthe tube-of Fig. 2. The relative 1 potentials associated with each path of travel depicted has been designated by plus and minus signs in the manner used in the description of Figs. 3A through 3D.

The beam. emitted by cathode 42A passes uninterrupted along the singular equipotential surface associated solely with the linear array of elements 41A to the collector 43A. This corresponds to the interconnection of a circuit .associatedwith cathode 42A to one associated with collector 43A.

The beam emitted by cathode 42B, however, travels along a singular equipotential surface which is associated initially with the linear array 41B and but subsequently becomesassociated with the linear array 41C until finally collected by the collector 430. This shift is effected by a change in potentials of an element associated with linear array 41B, an element associated with linear array 44B, and also one associated with linear array 41C. Such a path corresponds to the interconnection of a circuit associated withthe cathode 423 with a circuit associated with the collector .430. I

'The beam emitted by cathode 42C launched initially on one of the two singular equipotential surfaces associated with the linear array 41C is made to return along the other of the two singnllar equipotential surfaces until collected by collector 47C which is one of an array of collectors 47A through 47E positioned adjacent the source end of each linear array of elements 41A through 41E butotherwise of the kind used for collectors 43A through 43E. Each of these collectors is similarly positioned so as to intercept a beam flowing along a singular equipotential surface back towards acathode and yet not to impede the flow of, electron beams emitted by the cathodes in the direction towards collectors 43A through 43E.

The beam emitted by cathode 42D is shown with a path of flow which is initially along a singular equipotential surface associated with linear array 41D but subsequently doubles back towards the cathode end of the tube along a singular equipotential surface associated with linear array 41E until finally collected by collector 4713. To. provide such, a trajector an element in each 6 of linear arrays 41D, 44D and 41B has its potential modified in the manner illustrated.

.It should be .evident at this point that there isconsiderable flex'ibilityin the path of flow which maybe provided to anyelectron beam inthe tubeillustrat'ed. It :should also be evidentthat any number of interconnections may be provided by appropriate choice of the number of linear arrays, cathodes and collectors. By appropriate choice in the number of elements in each linear array and a suitable control system, the beam emitted by any cathode can bedirected to anycollector. :Variousapplications of a tube of such properties will be evident to Workers in the switching and computing arts.

In Fig. 5,thereis.shown schematically, as-anotherillustrative embodiment of the principles of. theinvention, a portion of a binary-type switching tube 50. Hereiit will be convenient to show merely a fragment of a longitudinal cross sectionof thetube. Initially, at thesource end of the beam, a single linear array 51 of conductiveelements is used to set up a pair of singular equipotential surfaces along/one ofwhich istlaunched an electron beam emitted by cathode 52. A distance along the path of flow a fork is provided beyond which two possible paths defined by linear arrays 53. and 54 are available to the electron beam. Each of thelinear array of elements '53 and 54 is able to provide a singular equipotential surface which forms a continuous extension of the singular equipotentialsurface associated with the linear array 51 along which the beam is flowing. Continuation of the flow along one of the two available paths is controlled by the switching potentials applied to the element of each of lineararray of elements 53, 54 which isv proximate to the linear array '51. In the tube illustrated, the path of flow depictedcorresponds to the case in which ele ment 53A is maintained at the same positive potential as that on the elements .of the array 51, which has the effect of merging without discontinuity the singular equipotential surfaces of. linear arrays 51 and 53 .while element 54A is rnaintained ata potential negative to :that on the elements of the array 51, which has the effect-of introducing a discontinuity between the singular equipotential surfaces provided by the linear array 51 and the remainder of the array 54. As a result, the beam cont nues its way only along the branch path associated with the linear array 53.

Moreover, further branching of the beam path can be provided by additional forks of the kind described along each of the paths associated with linear arrays ;53 and 54, as shown by arrays 55, 56, 57 and 58. It should be evident that such branching may be continued as often as necessary to provide a desired number of binary switchmg digits. There is associated with each possible path a target electrode (not shown) which serves to collect the beam. Accordingly, interconnection may be provided between a circuit associated with the cathode 52 and the circuit associated with any particular collector under the control of switching intelligence applied to-fix the potentials of the first elements of each linear array at a fork or switching region.

The principles of the tube described above can be extended to a decimal type of switching tube 60 of the kind shown in fragmentary form in Fig. 6. In this instance, an electron source 61 provides an electron beam which is initially directed past a longitudinal linear array of conductive elements 62, 63 and 70 through 79 on a singular equipotential surface associated with such array. Elements 70 through 79 of the array are associated with different decimal digits zero through 9, respectively, the last digit of the reference numeral denoting the digit. Associated with each digit, there is a transversely extending array, designated 80 through 89, respectively. It is to be noted that arrays 80 through 89 are substantially colinear with elements 63 through 78, respectively, since the application of a negative potential on one of the elements 70 through 79 prevents the beam from reaching require only a single control terminal.

that element but rather diverts it to flow past the trans-' verse array associated with the immediately preceding element. Each of the arrays, 80 through 89 provides ten switchingpositions and'includes a collector element asso ciated therewith. Such a two stage arrangement can han dle one hundred switching positions correspondingto one transferring the electron beam from said one to another hundred collector elements designated 100 to 199, the last two digits of such reference numerals'corresponding to the switching position, and requires but two control pulses.

' The first pulse is usedto control the extent to which the beam travels past the linear array 70 through 79 and hence fixes the tens digit in the switching position. The second pulse is used to control the extent to which the beam travels along the linear arrays 80 through 89' f tens digits and ten for the units, digits.

There is shown'illustrated by the solid line the bea nal linear arrayis made negative with respect to, the other elements of the array diverting the beam into the co'rre' sponding branch'path formed by linear array 85. a More.-

over, the second control pulse is used to make negative each element corresponding'to the unit digit of arrays 80 through 89, including array 85, whereby the electronibearn flowing alongone of the singular equipotential surfaces associated with array 70 through 79 is reflexed and diverted to the other singular equipotential surface whereby, it impinges on the collector electrode 150.

Itis to be understood that the various arrangements.

, described are illustrative of, the, general principles ofthc I invention. ,Various other'embodimentsmay be devised' by one. skilled in the electronics art without departing from the spirit and scope of the present invention.

What is claimed is:

1. In an electron discharge device, in combination, means defining a spaced pair of conductive boundaries, a linear array of conductive elements which are positioned between said boundaries and maintained at a potential difference with respect thereto for establishing a pair of singular equipotential surfaces, means for injecting an electron beam with a velocity for flow substantially along one of said surfaces, and means for changing under the influence of control information the r potential on at least one of said elements relative to the potential on other of said elements for modifying the path of said one surface and in turn the path of electron flow.

2. In an electron discharge device, in combination, means defining a spaced pair of conductive boundaries, a longitudinal array of conductive elements which are positioned between said conductive boundaries and are maintained at a positive potential with respect thereto for establishing a pair of singular equipotential surfaces, means for injecting an electron beam on one of said surfaces with a velocity for flow substantially therealong, and means for modifying the potential on at least one of the elements relative to the potential on other of said elements for modifying the path of said one singular equipotential surface and changing the path of electron flow correspondingly.

3. In an electron discharge device, in combination, means defining a spaced pair of conductive boundaries, a plurality of conductive elements arrayed between said boundaries and maintained at a potential difference with respect thereto to establish a plurality of singular equipotential surfaces, an electron source for injecting on one of said singular equipotential surfaces an electron beam with a velocity for stable flow therealong, and control means under the influence of applied intelligence for 7 elements for controlling the pathsof. the singular equipath corresponding to switching position 50. The ele' ment 75 corresponding to the tens digit of longitudisingular equipotential surface,

4. In an electrondischarge device, in I plurality of conductive elements arrayed for establishing a plurality of singular equipotential surfaces, meansdefining a pair of conductive boundaries on opposite sides of each array of elements, means for injecting an electron beam on one, of ,said'surfaces comprising an electron source and an acceleratinganode, means including voltage supply means for establishing on selected elements a potential suitably positive with respect to the electron source and the boundary defining means and for maintaining the accelerating anode at the potential of the singular equipotential surfaces with respect to the electron sourcejand means for varying the potential on elements,

of the array relative to the potential on other of said potential surfaces. 7

v 5. In electron beam apparatus, an electron beam source and ajtarget spaced apart, means defi ning a spaced pair of conductive boundaries, ,an array'of conductive clements which are positioned between said'bou'ndaries and maintained at a potential difference with respect thereto for forming a singular equipotential surface extending between said source and target, means for injecting an electronjbearn from said source on said surface with a velocity for 'fiow substantially therealong, and means for varying the potential of at least one of the elements of the array relative to the potential on other of said elements for vary-t ing under the influence of controlinformation the path of said surface for changing the path of electron flow pair of conductive boundaries, a longitudinalv array of conductive elements which are positioned between said boundaries and maintained at a potential difference with respect thereto for establishing a pair of singular equipotential surfaces,means for injecting an electron beam along one of said: surfaceswith a velocity for flow sub- I stantially therealong, means spaced apart along said surface for collecting the electron beam, and means for varying in turn the potential on at least a pair of spaced elements of the array relative to the potential on other of said elements for temporarily transferring the electron beam from flow along said one to the other of said pair of surfaces for varying the time of transit of the electron flow between said injecting means and collecting means.

7. In electron beam apparatus, an electron source and a plurality of target electrodes spaced apart from the source, means defining a spaced pair of conductive boundaries a plurality of conductive elements arrayed between the source and each target electrode and positioned between said boundaries, said elements being maintained at a potential difference with respect to said boundaries for establishing a plurality of continuous singular equipotential surfaces between said source and target electrodes, and means under the influence of control information for selectively interrupting all but one singular equipotential surface extending between said source and a selected target.

8. In switching apparatus, an electron discharge device comprising a plurality of electron sources and a plurality of collecting electrodes spaced apart, means defining a spaced pair of conductive boundaries and a two dimensional array of conductive elements positioned between said boundaries, at least some of said elements being maintained at a potential diflerence with respect to said boundaries for establishing selectively under the influence of control information singular equipotential surfaces, each interconnecting uniquely an individual electron source with an individual collecting electrode.

9. In an electron discharge device, in combination, means defining spaced conductive boundaries, means including a plurality of conductive elements positioned between a pair of said boundaries and maintained at a potential difference with respect thereto for forming a main combination, a I

singular equipotential surface, means including a plurality of conductive elements for forming a plurality of auxiliary singular equipotential surfaces, means for injecting an electron beam for flow along the main singular equipotential surface, and means including means for varying the potentials of selected elements of said pluralities of conductive elements relative to the potential on other of said elements for selectively providing continuity between the main surface and an auxiliary surface for diverting the electron beam from flow along the main surface to said selected auxiliary surface.

10. An electron discharge device comprising means for defining an electric field characterized by a plurality of equipotential surfaces, said means including an array of spaced electrodes and conducting means on each side of said array and spaced therefrom, means for applying potentials to said spaced conducting means and to said array such that at least one of said equipotential surfaces winds between successive electrodes of said array, means including a source of electrons and accelerating means for injecting electrons substantially onto said one equipotential surface at a Velocity to cause said electrons to travel substantially along said one equipotential surface, and conrol means for altering the potential on at least one of said spaced electrodes in said array relative to the potential on other of said electrodes for modifying said one equipotential surface and in turn altering the path of electron flow.

11. An electron discharge device comprising means for defining an electric field characterized by a plurality of equipotential surfaces, said means including an array of spaced electrodes and conducting means on each side of said array and spaced therefrom, means for applying potentials to said spaced conducting means and to said array such that at least one of said equipotential surfaces winds between successive electrodes of said array, means including a source of electrons and accelerating means for injecting electrons substantially onto said one equipotential surface at a velocity to cause said electrons to travel substantially along said one equipotential surface, a collector electrode for collecting said electrons after travel along said equipotential surface, and control means for altering the path of flow of electrons from said source to said collector comprising means for altering the potential on at least one of said spaced electrodes in said array relative to the potential on other of said electrodes for modifying said one equipotential surface in accordance with applied intelligence.

References Cited in the file of this patent UNITED STATES PATENTS 2,143,146 Farnsworth et al Jan. 10, 1939 2,179,996 Farnsworth et al Nov. 14, 1939 2,207,354 Pierce July 9, 1940 2,414,121 Pierce Jan. 14, 1947 2,513,260 Alfven et al June 27, 1950 2,711,478 Krawinkel June 21, 1955 

